The present invention relates to improvements in apparatus for manufacturing discontinuous cross-sectional structural board from granular materials.
In the manufacture of discontinuous cross-sectional structural boards from granular material, it is conventional to use apparatus as described in U.S. Pat. No. 3,229,009, issued Jan. 11, 1966, entitled "METHOD AND APPARATUS FOR FORMING COMPOSITION BOARD" and U.S. Pat. No. 3,142,185, issued July 28, 1964, entitled "PISTON STROKE ADJUSTMENT". In apparatus of this type, particulate material, such as various types of wood sawdust, wood chips, wood scraps and the like, which have been comminuted, are used in the formation of structural members. The particulate material is conventionally mixed with thermosetting adhesive and is then forced through a heated mold. In molding these products, economic factors make it desirable to obtain as high a rate of production as possible while maintaining the uniformity of the finished board.
In the extrusion process, the density of the product is partially controlled by surface friction on the material as it is forced through the mold cavity. Thus, any distortion of the mold cavity, either from internal pressures, external forces or distortion of mold sections, can vary the surface friction and thus produce a product of uneven density. Distortion in the mold cavity can likewise result in a distorted finished product. During extreme cases, distortion, of the mold cavity can prevent operation of the molding process.
Therefore, one of the problems encountered in designing equipment for molding particulate material is that of producing a molding system where distortion of the mold cavity is minimized. To achieve this goal, it is necessary to minimize, or eliminate, differential expansion within the mold due to variations in operating temperature. It is therefore, important to not only maintain a uniform temperature within the mold assembly itself, but to locate the heated mold from frame members which surround the mold assembly and supply the compressional forces exerted on the materials during the extruding process.
Machine frame distortions caused by heat conducted from a heated mold assembly can cause a drastic distortion of the mold cavity. The massive head and base members surrounding the heated mold assembly are subject to becoming hotter in areas close to the mold than in the more remote areas. For example, when heat from the mold is conducted into the adjacent lower portions of the upper frame member, the temperature in the lower portion becomes higher than that in the top portion. This causes a lateral expansion in a hotter lower portion causing the frame member to bow downward in the center between the side supports. A similar distortion occurs in the frame member adjacent the lower mold section. This uneven expansion results in tremendous forces against the midsection of the top and bottom mold sections, causing them to deform and thus reducing the uniform thickness of the mold cavity. While the internal forces from molding the granular material acts opposite to the inwardly directed force caused by the uneven expansion, these forces will not prevent the deformation of the mold cavity.
This distortion not only interfers with the entry of the plunger into the mold cavity but also causes a thinner board section toward the center of the cavity mold. It further alters uniform feeding and frictional forces on the surface of the material passing through the mold cavity.
Therefore, there exists a need for an improved molding apparatus capable of minimizing the mold distortion due to heat gradients. The present invention provides an improved mold apparatus which is provided with spaced bars for connecting the frame to the mold and in which the spacer bars are provided with longitudinal prongs for engaging the mold surface. The faces of the prongs contacting the mold surface are of sufficient area so that molding pressures will not cause the prongs to be embedded in the mold surface while being sufficiently narrow to limit the area contact between the mold and the bars. The remaining surfaces of the mold and the surfaces of the spacer bars created by the prongs thus become radiating surfaces to dissipate heat conducted into the spaced bars. The outer face of the spacer bars are likewise slotted to reduce the contact area in which the bars engage the frame members. The surface areas between these longitudinal slots thus become additional radiating surfaces to further reduce heat conduction into the frame.
It is preferable that the prongs on one surface of the spacer bar and the extensions on the opposite surface between the slots are staggered in lateral relationship to minimize the direct paths for heat conduction from the mold sections to the frame. The remaining center section of the spacer bars are cored for water or other coolant circulation. Thus, by cooling the solid sections between the prongs bearing on the mold and the portions in contact with the frame, heat conducted from the mold into the frame is effectively reduced or eliminated. In addition, heaters are formed in the mold adjacent the peripheral surfaces thereof and are operated to maintain the periphery of the mold at a constant temperature. These longitudinally spaced resistance heaters are installed close to the outer surfaces of the mold assembly. These heaters are conventionally placed immediately under each prong of the spacer bars to thus reduce heat differential within the mold.
A plurality of longitudinally extending heaters are positioned near the edge of the molds forming the mold cavity. Thermocouples are likewise positioned adjacent the surface of the mold forming the mold cavity and function to control the heaters adjacent thereto. The lineal capacity of these heaters vary along the length of the mold cavity in proportion to the heat requirements of the product moving through the mold and the proximity of the heaters to the ends of the molds.